Selected Quarterly Data - Summarized Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 14,780	$ 12,639	$ 12,549	$ 12,687	$ 11,825	$ 10,239	$ 9,546	$ 9,656	$ 52,655	$ 41,266	$ 52,120
Net loss	$ (14,588)	$ (12,481)	$ (12,407)	$ (12,602)	$ (11,734)	$ (10,178)	$ (9,468)	$ (9,589)	$ (52,078)	$ (40,969)
Basic and diluted net loss per share (usd per share)	$ (0.35)	$ (0.30)	$ (0.29)	$ (0.39)	$ (0.37)	$ (0.32)	$ (0.30)	$ (0.31)	$ (1.31)	$ (1.31)	$ (2.07)

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

No definition available.

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration